Regulation and Function of Extracellular pH in Shaping Molecular and Developmental Landscapes

Abstract

Diese Dissertation ist gesperrt bis zum 31.08.2028!

The concentration of protons (H+), commonly expressed as the power of hydrogen (pH), is of paramount importance to all forms of life, as proton concentrations modulate the physicochemical properties of virtually all biomolecules. Thus, it is essential for cells to maintain and tightly regulate optimal subcellular pH values, which specify organelle function and modulate cellular activity. In multicellular organisms, cell-to-cell variation in intracellular and extracellular pH correlates with organ development. Nevertheless, how multicellular organisms establish precise pH patterns and the extent to which these patterns may instruct development remain largely unknown. In this thesis, I investigated the molecular mechanisms underlying extracellular pH (hereafter pHe) patterning in Arabidopsis thaliana (hereafter Arabidopsis) root and uncover an ancient alkalinization mechanism that links cell-surface processes, pH homeostasis, and the modulation of developmental transitions. In Arabidopsis root, the meristematic and differentiation zones are distinctively alkaline. Although plasma membrane H⁺ ATPases are major determinants of pHe in plants, their transcripts and protein accumulation do not correlate with pHe along the longitudinal root developmental axis. By using cell-to-cell variation in transcript accumulation of the Arabidopsis H⁺ ATPases 2 (AHA2) as a probe, this work identifies members of the REMORIN (REM) protein family as candidate regulators of plasma membrane H⁺ ATPases. Live-cell imaging and biochemistry approaches showed that group 1 REM proteins act as negative regulators of proton pump activity, promoting extracellular alkalinization. We show that REMORIN1.2 (REM1.2) accumulation marks alkaline zones of the root at both cellular and organ levels. We show that, REM-mediated pHe modulation regulates diverse pH-sensitive processes at the cell surface, including hormone signaling, endocytosis, cell wall chemistry, and ultimately root developmental transitions and growth. We showed that REM act as negative regulators of plasma membrane H⁺ ATPases in the liverwort Marchantia polymorpha (hereafter Marchantia), indicating that this function is conserved in land plants and predates the emergence of true roots. Finally, structural modelling and bio-assays indicate that REMs bind to 14-3-3 proteins to regulate plasma membrane H⁺ ATPases, observations which provide a basis to infer how REMs regulate pHe. The work engaged during my doctoral thesis show that the role pHe in development goes beyond the established acid-growth theory, it identifies an evolutionarily conserved regulator of plasma membrane H+-ATPases and places pHe regulation as a putative driver of the evolution of complex body structure. I discuss the implications of these findings for other systems and biological contexts and present my contributions to collaborative studies on additional roles of pH in development and immunity.